What moral lines can and can't we cross as a society?

From Amazon: "Should we pay children to read books or to get good grades? Should we put a price on human life to decide how much pollution to allow? Is it ethical to pay people to test risky new drugs or to donate their organs? What about hiring mercenaries to fight our wars, outsourcing inmates to for-profit prisons, auctioning admission to elite universities, or selling citizenship to immigrants willing to pay? "